MEMS (Micro electro mechanical system) devices may be used in many applications including rear and front projection scanned beam displays, scanned beam image capture devices, optical gyroscopes, accelerometers, and other applications. In addition to displays that project an image onto a conventional opaque or translucent viewing screen, scanned beam displays can include retinal scanning displays (RSDs) and heads-up displays (HUDs). Scanned beam image capture applications include one-dimensional (1D) or linear scanning devices such as linear bar code scanners and two-dimensional (2D) image capture devices such as 2D bar code or omnidirectional linear bar code imagers, 2D bar code scanners, confocal microscopes, microprobes, medical imaging systems, and others.
For cases where the MEMS device is used to scan a beam of light, it is frequently called a MEMS scanner or beam deflector. MEMS scanners may operate resonantly or non-resonantly, and may scan in one or a plurality of axes.
MEMS devices may carry light emitters directly or alternatively may deflect a beam through a scan angle. In beam deflection applications, one or more scan plates have a reflective surface that is used to scan an impinging beam over a field of view. The reflective surface may include a plated reflective metal such as gold or aluminum, a dielectric stack, bare silicon, or other materials depending upon wavelength and other application issues.
2D scanning may be achieved by arranging a pair of 1D scanners with their axes of rotation at substantially right angles to one another. Alternatively, 2D scanners may use a single mirror that is driven to rotate around both scanning axes. When a single mirror is used to scan in two axes, a gimbal ring may be used to allow appropriate rotation. Frequently, 2D scanners include an inner scan plate carrying a mirror that performs a fast scan with an outer gimbal ring performing a slow scan. Conventionally, the fast scan sweeps back and forth horizontally across the field of view (FOV) while the slow scan indexes down the FOV by one or two lines. Such systems may be termed progressive scan systems. In such systems the fast scan operates at a relatively high scan rate while the slow scan operates at a scan rate equal to the video frame rate. In some applications, the fast scan operates resonantly while the slow scan provides a substantially sawtooth pattern, scanning progressively down the frame for a (large) portion of the frame time and then flying back to the top of the frame to start over. In other applications, interleaved sawtooth scanning, triangular wave scanning, sinusoidal scanning and other waveforms are used to drive one or both axes.
Although this document frequently refers to a fast scan direction as horizontal (rotating about a vertical scan axis) and a slow scan direction as vertical (rotating about a horizontal scan axis), it must be realized that such a convention is not limiting. The teaching applies similarly to systems with fast and slow scans in the vertical and horizontal directions, respectively, as well as other directions.
In progressive scan systems, the beam may be scanned unidirectionally or bidirectionally depending upon the desired resolution, frame rate, and scanner capabilities. Bi-directionally scanned systems may suffer from raster pinch as described by Gerhard et al in U.S. Pat. No. 6,140,979 entitled Scanned Display with Pinch, Timing, and Distortion Correction. One approach to compensating for raster pinch is to add a correction mirror that corrects the beam path to more nearly approximate an ideal raster pattern.
More recently, work by the applicant has focused on alternative scan patterns that scan the beam in a Lissajous scan pattern over the FOV. Lissajous scan patterns have an advantage in being able to operate the MEMS scanner resonantly in both axes, thus reducing power consumption. Such systems may also have reduced torque requirements and may thus be made smaller and have other advantages.
Various actuation technologies for MEMS scanners have been disclosed. Electrocapacitive drive scanners include both rear drive pad and comb drive architectures. Magnetic drive scanners include moving coil and moving magnet types. Other technologies include thermal, piezoelectric, and impact motor drives. Rotation may be constrained by torsion arms, bending flexures and other arrangements. Electrocapacitive drive systems are sometimes referred to as electrostatic in the literature. Bending flexures are popularly referred to as cantilever arms.
Frequently, two or more drive schemes are combined to provide independent drive in two or more axes. For example, the Gerhard et al patent listed above shows a MEMS scanner with a fast scan axis that is powered electrocapacitively and a slow scan axis that is powered magnetically. The need to provide independent drive actuators for each axis has heretofore limited size reductions as well as the number of axes.
Another aspect of MEMS oscillator requirements frequently includes the need to monitor device motion or angle. Various schemas have been proposed and used including piezo-resistive and optical feedback.